1. Field of the Invention
The present invention relates generally to an end position cushioning system for a double acting pneumatic cylinder and more particularly to a cushioning system using a volume chamber of air to provide reduced noise cushioning.
2. Brief Description of the Prior Art
A double acting cylinder typically includes a piston that reciprocates inside a cylinder tube. The piston is generally coupled to a slide or follower that moves in conjunction with the piston. The piston is reciprocally moved pneumatically in the cylinder by injecting a fluid, usually air, into the ends of the cylinder. The movement of the piston can be stopped by can be use a number of different methods such as a bumpers or air cushions as discussed below.
When the piston comes into contact with the end cap it is possible that the end cap or piston may be damaged by the piston engaging the end cap. In addition, when the piston hits the end cap there is the possibility that the follower attached to the piston may stop abruptly. Thus, the movement of the follower in such situations is not very accurate and may transfer unwanted vibration to the object attached to the follower. Furthermore, when the piston contacts the end cap, a loud noise may be created since the piston and the end cap are usually made of rigid materials. Currently there exist several cushioning methods to address the problem of providing cushioning between the piston and the end cap so that the piston will not contact the end cap and make noise.
The first type of cushioning is a fixed bumper system 40 as shown in FIG. 2. In the fixed bumper system, the surface of the end cap 42 that contacts the piston 44 includes a cushion 46 in the form of a shock absorbent material. This shock absorbent material may be a spongy or rubber type material so that when the piston 44 comes into contact with the end cap 42, the fixed bumper 46 will cushion the impact of the piston 44 hitting the end cap 42. The shock absorbing material brings the piston to a halt and decreases the amount of noise that is produced when the piston reaches the end of its stroke. The fixed bumpers are commonly made from a synthetic rubber such as polyurethane or neoprene since they are both durable and provide good shock absorption. In addition, these materials are relatively cheap and not only help relieve the impact forces but also reduce the amount of noise that is produced when the piston hits the end cap.
However, the disadvantage to using the fixed bumper is that over time the shock absorbing material will wear out from the constant use of the piston crushing the bumper. Another disadvantage to the fixed bumper cushioning system is that since the fixed bumpers are inherently a spongy type material, the compression rate of the material may change over time as it is compressed. Thus, the stroke distance that the piston will travel will vary slightly due to the changing compression rate of the fixed bumper as it is used.
A second type of cushioning system is an adjustable air cushioning system as shown in FIGS. 3 and 4. In the adjustable air cushioning system 50, a piston spear extends outwardly from the piston body and engages a volume chamber within the end cap. In the adjustable air cushion 50, as the piston 52 approaches the end cap 54 the air in the volume chamber between the piston spear and the end cap is compressed and released via a small orifice 58 in the end cap 54. In addition, a needle valve 56 is inserted within the path of the exhaust to further regulate the rate at which the trapped air evacuates (FIG. 4). More specifically, the needle valve may be adjusted to control the rate at which the air is released thereby regulating the end of the piston stroke. Thus, the piston can be brought to a stop without contacting the end cap by slowly letting air evacuate the volume chamber through the needle valve. An advantage of this cushioning method is that the impact stresses are reduced and cushioning is more efficiently achieved when compared to fixed bumpers. This method of cushioning is also more expensive than the fixed bumper since an additional component is required, i.e., a needle valve. Furthermore, should the needle valve fail to operate, the piston will contact the end cap possibly causing severe damage to the device. Like fixed bumpers, the accuracy of the piston movement may be variable due to the changing compression properties of the air used in the cushioning system.
Still a further known type of cushioning system is not to use a cushioning system at all between the piston and the end cap, but rather use an external shock absorbing system. This system may take the form of small shock absorbers which use dampers or simple rubber shock absorbers attached to the follower housing. The disadvantage of this system is that there is still a significant amount of noise produced and the external cushion or external shock absorbers are expensive and require continued maintenance.
It would be desirable to create a cushioning system that overcomes the disadvantages of current systems such as high impact forces created by the piston coming into contact with the end cap. It would also be desirable to create a cushioning system that reduces or completely eliminates the noise produced by the piston coming into contact with the end cap or the slide contacting an external shock absorber.
It would also be advantageous to create a cushioning system that completely eliminates the piston from coming into contact with the end cap thereby greatly reducing the amount of wear and noise generated by the piston reaching the end of its stroke.
It is an object of the present invention to provide a cushioning system that relieves the impact forces associated with a double acting cylinder piston reaching the end of its stroke.
It is another object of the present invention to provide a cushioning system that greatly reduces or eliminates the noise produced by the double acting cylinder piston reaching the end of its stroke.
It is still a further object of the present invention to provide a cushioning system including a secondary cushioning system should the primary cushioning means should fail.
In one form of the present invention a pneumatic cylinder having a cylinder tube with at least one open end may be coupled to an end cap to close the open end. The pneumatic cylinder may have a piston that is reciprocally movable in the cylinder. The piston has a first seal to create a substantially fluid-tight seal with the internal surface of the cylinder tube. In the preferred embodiment the piston has a piston spear at the end portion of the piston for cooperating with a piston spear receiving space in the end cap. Preferably, there is a second seal in the piston spear receiving space so that when the piston spear contacts the second seal a substantially fluid-tight seal between the piston spear and the second seal in the piston spear receiving space is created, thereby creating a volume chamber between the first seal and the second seal which is used as an air cushion at the end of the piston stroke to prevent the piston from contacting the end cap.
In a more preferred embodiment of the present invention it is envisioned that there is at least one elastomeric cushioning device positioned adjacent to the end surface of the end cap to provide a secondary cushion between the piston and the end cap. Preferably, this secondary cushioning may be made of neoprene and polyurethane. The advantage of the secondary cushion is that should the first or second seal which creates the volume chamber leak or fail, the piston would not contact the end cap. Instead, the piston would come in contact with the elastomeric cushioning device so that the piston would be safely stopped and not damage either the end cap or the piston.
Further objects and features of the present invention will better be understood in light of the embodiment examples which are discussed below with the aid of the accompanying drawing.